Electrical discharge lamp



Sept 18, 1955 H. E. KREFFT ET AL 2,763,803

ELECTRICAL DISCHARGE LAMP Filed March 23, 1955 nilite ELECTRICAL DISCHARGE LAMP informaron E. Krefft, East Orange, George M. Harris, selin, and Albert l?. Koerbis, Metuchen, N. J., assignors to Hanovia Chemical & Manufacturing Co., Newark, N. l., a corporation of New Jersey Application March 23, 1955, Serial No. 496,198

Claims. (Cl. 313-49) The present invention relates to electrical discharge lamps of the compact arc type, which are light sources of very high brightness operated under high pressures.

Compact arc lamps are usually composed of a spherie cal or nearly spherical discharge vessel of considerable wall thickness made from clear fused silica with a pair of elongated current carrying seals of tubular shape cxtendinfy therefrom and which are fused to the discharge vessel coaxially and at opposite ends. Each seal is provided with an operating electrode, the spacing between these electrodes being of the order of a few millimeters. ni'he discharge vessel, depending on the type of lamp contemplated, is filled with a rare gas, for example, xenon or krypton, under a pressure of several atmospheres at room temperature, or it contains a rare gas atmosphere and a limited amount of mercury which under operating conditions of the lamp forms a vapor atmosphere of high pressure usually exceeding 2O atmospheres. Under these conditions, when the lamp is operated from a source of current, an electrical discharge is produced between the electrodes which forms a short electrical arc of very high brightness. Compact arc lamps, therefore, require ccurately positioned support members by means of which they can be installed in a fixture, for instance, a searchlight retiector or a projector in a desired position so that the arc is in precise alignment with the optical system and lamps can be easily exchanged without requiring individual optical adjustment. The support members, therefore, must be well aligned with the arc and correctly located with reference to its center.

Usually, compact arc lamps are provided with metallic bases of tubular shape which are slipped over the seal ends and clamped or cemented to the seals. The lead-in conductors of the seals supplying current to the electrodes are electrically connected to the bases which serve two purposes simultaneously, that is, to support the lamp in the lamp holder and to supply current to the electrodes. This basing method, however, is not satisfactory as a clamped or cemented connection of the base to the seal is not reliable and may work loose; also, the seal is exposed to mechanical strain or contamination through the cement which often leads to cracks or devitriiication. ln these lamps, the seals consist of clear fused silica or similar vitreous material operated at elevated temperatures and the lead-in conductors are composed of highly oxydizable metals, therefore, any base through which heat dissipation from the seal is retarded may produce oxydation of the seal lead and devitrilication of the highly susceptible vitreous material of the seal body, thus shortening lamp life. Moreover, with bases of this type, accurate alignment and locating with respect to the arc is difficult as it can be accomplished only with special jigs and requires complicated base constructions.

ln order to overcome these difficulties, it has been suggested to support compact arc lamps by means of clamps which are firmly pressed onto the shaft of each seal away from the seal end and separated from the current carrying leads energizing the electrodes. However, through this arrangement facilitates the support of the lamp, good alignment, and correct locating of the supporting clamps is not achieved. Furthermore, clamping directly on the vitreous seal does not provide a mcchanically firm connection to the seal which at this point near the discharge vessel assumes high temperatures during operation of the lamp, thus weakening the hold of the clamp. Also, any clamped connection, if made suiciently firm, is liable to produce dangerous strain in the seal body. As in 'this way the seal end is not utilized for supporting the lamp, a base contact only is required which protects the lead-in conductor emerging from the seal and provides electrical connection to the circuit operating the lamp.

In view of these imperfections which affect the successful use of compact arc lamps, it is one object of the present invention to provide an improved lamp supporting fitting which is accurately located with respect to the arc and connected to the seal body in an unchangeable position without the use of a clamp or a cement, thus avoiding mechanical strain in or contamination of the seal. Another object is an improved base contact which can be easily assembled with the lamp seal without clamping or cementing. Other objects and advantages of this invention will become apparent from the description hereinafter following and the drawings forming part hereof in which:

Figure l is a side view of a lamp constructed in accordance with the invention,

Figure 2 is a sectional top view of a seal provided with a supporting tting and a base contact of the lamp shown in Figure l,

Figure 3 is a sectional view of a seal and supporting fitting taken on the line III- III of Figure 2,

Figure 4 is a sectional view showing a modification of Figure 3, and

Figure 5 is an end view of a base cap provided with cooling tins.

In accordance with this invention, an electrical discharge lamp comprises an envelope of clear fused silica or similar vitreous material lled with an ionizable atmosphere, a pair of elongated current carrying seals of similar vitreous material, each provided with a lead-in conductor, a pair of spaced electrodes supported by said seals, at least one annular supporting fitting connected to one of said seals, each of said seals comprising a cylindrical shaft portion adjacent to said envelope, a moulded portion, and an end portion, said shaft portion having an annular groove of substantially circular cross section and of smaller diameter than said shaft portion, said supporting fitting being composed of a split annular bushing seated in said groove and preferably fitting said groove with some clearance between the bushing and the surface of said groove, and a ring portion surrounding and firmly connected or fixed to said bushing, base caps rmly connected to said outer leads and covering or embracing said end portions of the seals, with some clearance therebetween, and flexible cable leads connected to said base caps. According to a further characteristic, the annular groove for the supporting fitting is provided with a plane portion, and one of the bushings has a plane profile fitting said plane portion and securing the supporting 'fitting against rotation.

Referring to Figure l of the drawing, a lamp constructed according to the invention consists of a spherical or nearly spherical discharge vessel 1 to which are fused coaxially and at opposite ends elongated seals 2 and 3 of which each contains a lead-in conductor carrying electrodes 4 and 5. Preferably, the discharge vessel is made from clear fused silica or a similar vitreous material of high softening point and its wall has a thickness of about three millimeters so that it will safely stand the high filling and operating pressures common in such lamps. The seals are made from the same materials and they are elongated cylindrical bodies with lead-in conductors embedded in their centers. Each lead-in conductor is composed of an outer lead 6 which emerges from the seal, an electrode supporting inner lead 8 and a substantially flat lead 7 interconnecting the inner and the outer leads. Preferably, the leads 6 and 8 are made from tungsten rods while the lead 7 is a thin molybdenum foil about '.002 inch thick. Interconnection between these three leads is produced by spot Welding and by means of a suitable soldering material like platinum. The molybdenum foil lead is vacuum tightly embedded in the vitreous material of the seal body and the outer lead 6 is firmly but not hermetically embedded, while the other leads 8 is preferably arranged within the seal body with a slight clearance so that no harmful strain is produced in the seal when the lamp is operated. Thus the body of a seal consists of three different parts, the seal shaft, which contains the supporting lead 8 and which is essentially circular in section, a flattened part which is vacuum tightly molded around the molybdenum foil '7, and an essentially round end part firmly molded onto the outer lead 6.

The discharge vessel is provided with a sealing tip 9 through which, prior to tipping off, the lamp is thoroughly degassed during the exhaust procedure and filled with a vaporizable material like mercury and/or a rare gas like xenon or krypton under a pressure of several atmospheres. As is customary in high pressure mercury lamps, and also in compact arc lamps to which the invention particularly refers, only a limited quantity of mercury is filled into the discharge vessel which is entirely evaporated under operating conditions of the lamp. When a source of electrical current, for instance an A. C. line voltage of 115 volts, is connected to the lead-in conductors of the lamp across a suitable ballast, an arc discharge is generated between the tips of the electrodes 4 and S which assumes a very high brightness and absorbs a considerable wattage thus heating the discharge vessel and the seals to high temperatures. For the mechanical support of the lamp, the seals are provided with annular fittings 10 which are connected to the seal shafts as shown in Figure l. The seal ends are protected by base caps 11 which are firmly connected to the rigid outer lead 6 and which carry flexible leads 12 through which the lamp is electrically connected to the operating circuit.

The invention is particularly concerned with the construction of the supporting fittings and of the base caps and with the means through which these parts are connected to the seals. As shown in Figures 2 and 3, the supporting fittings are arranged on the shaft portion 13 of the seal body which is provided with a groove 14, which has a smaller diameter than the seal shaft and which is advantageously produced by cylindrical grinding of the seal shaft with an abrasive tool; for instance, a diamond studed grinding wheel. This groove is well aligned with the axis of the seal and with the electrode supporting rod 8, and it is also positioned at a desired distance from the electrode tip which is thus preferably precisely located with respect to the groove. The supporting fitting consists of two concentric parts, an annular sleeve bushing, or ring, which is preferably split into two halves L15 and 16, which lit into the groove, and a ring 7 which is slipped over the bushing halves and thus holds these in the groove. groove 14 and the inner diameter of the bushing are such that there is only a small clearance between these two parts While the annular ring 17 through a screw 1S is iirmly connected to the bushing. The seal end is protected by a cap 11 which is slipped over the cylindrical end part 19 of the seal body and secured to the outer lead 6 by means of a screw 20 through which electrical connection is produced between the cap and the outer lead. This cap is also provided with a bore 21 which contains the end of a Lflexible v.cable lead 12 through which the lamp The diameter of the cylindrical d er:

may be connected to an operating circuit. A firm connection between the cap and lead 12 is produced by soldering, and in the same manner a reliable connection of good electrical and thermal conductivity is produced between the cap and the outer lead 6 by means of some soldering material 22.

As set forth and explained by the gures referred to, the supporting fitting for the lamp is connected to the seal shaft in an accurately located and unchangeable position without the application of any mechanical forces which would produce harmful strain and would not provide a durable connection as the seals assume high temperatures during operation of the lamp which tend to weaken any connection produced by such means. Since the bushing lits onto the groove with a slight tolerance of, for example, .001 to .002 inch the material of the seal is not exposed to any pressure of other forces, yet the fitting cannot change its position or hold the lamp seal. The bushing and the ring of the fitting are conveniently made from a non-corrosive metal, for example, stainless steel as these parts, and the seals of the lamp, assume elevated temperatures during operation of the lamp. Such materials have a higher coeflicient of thermal expansion than the clear fused silica of the seal body, therefore, no strain can be set up in the seal under any condition.

For the support of the lamp in a lamp holder which, for example, may have a V-shaped jaw accommodating the ring portion 17 of the tting, and a clamp securing it in a desired position, only one fitting is required but preferably both seals are provided with such fittings of which one is firmly held by to the lamp holder while the other one is utilized for the alignment of the lamp in the lixture and is conveniently held without clamping tightly, thus allowing for thermal expansion of the lamp holder relative to the lamp. This second tting may, therefore, be made shorter and smaller than the other. The alignment of the lamp, however, may be also effected by means of the end cap of the opposite seal. The construction of the supporting fitting illustrated by the drawings is shown by way of example only as there are other ways possible for manufacturing and for assembling it; for example, the ring portion 17 may have a much thinner wall than shown and may be pressed onto the bushing halves, or a soldered connection may be used instead of the screw 18.

Instead of providing a flexible lead 12 for supplying current to the seal, the base cap 11 may be used as a contact, however, providing flexible means for this purpose is advantageous as any clamped or screwed connections applied to the cap or the seal end may produce harmful strain. This cap also improves heat dissipation from the seal end as it is in good thermal contact with the outer lead 6 which in turn, owing to its relatively large cross section, conducts heat away from the foil 7.

While the supporting fittings, as set forth, have precisely located and unchangeable positions on the seals, they still can be rotated with respect to the axis of the lamp as they are not secured by clamping, and since their component parts and the groove provided in the seal body have circular cross sections. In many uses a possible rotation of the lamp around its principal axis which coincides with the arc axis is not objectionable and may even be desirable, but where a defined angular position of the lamp is desired this can be achieved by a simple modification of the construction already described. As illustrated by Figure 4 which shows one possible way for carrying out this modification, the groove on the seal shaft 13 is provided with a plane portion 23 which is conveniently produced by grinding of the seal on a universal grinder after performing the cylindrical grinding of the groove which thus consists of a cylindrical portion 24 and a plane portion 23. as shown in the cross-sectional View of the drawing. In accordance with this profile, the half bushing 15 has a plane bottom portion 25 which lits onto the plane surface of 23 with a small clearance, while the other half bushing 16 is circular and adapted to the cylindrical portion 24. When both bushing halves are assembled and held in position by the ring 17, the supporting fitting cannot be rotated owing to the locking action of the plane surfaces 23 and 25. The same result may be obtained by substituting a separate flat key for the bottom portion 25. In this case, the circular half bushing is provided with a slot cut into its wall, which slot accommodates part of the key piece. It may be also desirable to provide two parallel plane portions arranged on opposite sides of the groove and to use two equal bushing halves of which each has a plane bottom portion 25.

The base cap 11, besides providing mechanical protection for the seal end and electrical connection to the operating circuit of the lamp, also serves the purpose of dissipating heat which is conducted from the discharge vessel through the seal and generated in the thin molybdenum foil lead which possesses an appreciable electrical resistance. As molybdenum is a highly oxydizable metal, and the foil end connected to the outer lead 6 is to some degree exposed to atmospheric oxygen, the seal end must not exceed a temperature of 300 C. and it should stay well below this limit for otherwise the lamp would become inoperative after a relatively short time through interruption of the lead-in conductor. In a seal constructed according to Figure 2, this danger is greatly reduced as the tungsten rod forming the outer lead 6 is a good thermal conductor which carries away heat energy affecting the exposed foil end and communicates this energy to the base cap which acts as a cooling surface. For this reason, a good thermal contact between the outer lead and the cap, as produced by the soldered connection 22, is very essential. Moreover, as in the construction of seals of this type, it is advantageous to use tungsten rods which are highly polished by centerless grinding, only a very small clearance exists between the rod and the vitreous material of the seal end which is molded around the rod, thus only `a small quantity of air may have access to the foil lead, and oxydation even under temperatures near the dangerous limit, is therefore slowed down very appreciably.

The use of a firmly embedded tungsten rod as an outer lead of the seal has the further advantage that it has sufcient mechanical strength for the support of the base cap which in known lamp construction is clamped and/or cemented to the vitreous body of the seal end. A connection of this kind, however, is harmful to the seal as it is bound to produce strain and to prevent heat dissipation. With the present construction, this danger is avoided as the cap can be made very low and sufiiciently wide to provide a small clearance between its inner wall and the body of the seal end. Advantageously, the base cap is provided with cooling fins 26, as illustrated by Figures 2 and 5, through which its surface area may be considerably increased and heat dissipation from the seal end further improved.

The scope of the invention is not limited to electrical discharge lamps of the compact arc type of which one embodiment having two seals is described by way of example only, and it will be apparent to those skilled in the art that it may be also used on other evacuated or gas filled devices employing vitreous materials and particularly clear fused silica in their construction where the basing and supporting method described may offer many advantages.

What we claim is:

l. An elongated vitreous body, a circumferential groove in said body, a first sleeve member seated in said groove and a second sleeve member concentrically fixed to said first sleeve member.

2. A substantially cylindrical vitreous body, a co-axial shaft member passing through said body, a circumferential groove in said body, a first sleeve member seated in said groove and a second sleeve member concentrically fixed to said first sleeve member.

3. A seal member for electrical discharge lamps comprising a substantial cylindrical vitreous body, an electrical conductor co-axial with and passing through said body, a circumferential groove in said body, a first sleeve member seated in said groove and a second sleeve member concentrically fixed to said first sleeve member.

4. A seal member according to claim 3, wherein said sleeve members are inner and outer rings.

5. A seal member according to claim 3, wherein said sleeve members are inner and outer rings, said inner ring being a split ring.

6. An electrical discharge device comprising a vitreous envelope defining a discharge chamber, an ionizable atmosphere in said discharge chamber, a pair of elongated seal members, said seal members each comprising a substantially cylindrical vitreous body with one end thereof fused to said envelope, an electrical conductor co-axial with and passing through each of said elongated vitreous bodies into said discharge chamber and having ends in spaced relation to each other within said discharge chamber, a circumferential groove in at least one of said elongated vitreous bodies, a first ring member seated in said groove and a second ring member concentrically fixed to said first ring member, a base member on the free ends of said elongated bodies.

7. An electrical discharge lamp according to claim 6, wherein said base member is a cap secured directly to an end of said conductor.

8. An electrical discharge lamp according to claim 7, comprising spaced fin members on said cap member.

9. An electrical discharge lamp according to claim 7, comprising a lead-in conductor connected directly to said cap.

10. An electrical discharge lamp according to claim 6, wherein said electrical conductor comprises a pair of axially spaced rods connected by means of another electrical conductor hermetically embedded in said elongated vitreous body.

No references cited. 

